1. Field of the Invention
This invention relates to means for determining the firing instant of an air gun such as is commonly used in marine seismic surveying for sub-ocean economic minerals, U.S. Class 181/107 and/or 181/120.
2. Discussion of the Prior Art
Air guns have become a standard sound source for marine seismic reflection and refraction exploration. Used singly or in arrays, air guns are compact, economical to operate and versatile as to tailoring of the power and spectral energy content of the acoustic signal to environmental needs.
Accurate computation of the depth of subsurface earth layers is critically dependent upon an accurate measurement of the travel time of a reflected or refracted acoustic wave between the source, such as an air gun, and the acoustic-wave receivers, such as hydrophones. Accordingly, the instant of firing an air gun must be khown with a precision that is greater than the data-sampling-rate error, assuming of course that digital data recording is employed.
In principle, all air guns employ some sort of valving means to abruptly release a volume of high-pressure air or other compressible fluid from a chamber, through exhaust ports, into the surrounding water. The firing instant of the gun is measured in various ways. In a primitive method, a pressure detector is fastened two or three feet away from the exhaust ports. When the gun is fired, the resulting near-field shock wave is sensed by the pressure transducer as a sharp pressure increase. That method has the disadvantage that the location of the pressure transducer relative to the exhaust ports is seldom known with exactitude. Therefore, there is an unknown time delay between the time the gun fires and the time that the shock wave is sensed. Another disadvantage is that the shock wave often damages or destroys the active element of the detector which is usually of the piezo-electric type.
The air-release valve of air guns used in seismic operations is generally electrically triggered. Although at first glance, one would think that the trigger pulse itself might be used as the fire-time instant (time break), mechanical variations in the gun itself such as dimensional tolerance variations, friction, etc., cause the use of the trigger pulse as a time break to be unreliable. That problem is particularly serious when an array of guns must be fired in synchronism.
Another type of time break detector involves use of an electromagnetic or electrostatic means mounted on or in the gun to detect motion of the valve member itself. Such detectors provide an electrical transient when the valve member just begins or just ends its travel to uncover the exhaust ports. I have discovered that installation of such a device, usually inside the gun, requires extensive modifications to the gun body and it tends to be somewhat complicated. Field maintenance is difficult and costly because the entire gun must be disassembled to make repairs. Furthermore, the time that the gun fires is not necessarily coincident with the beginning or ending of the valve motion. The valve members of certain types of guns may be made of a non-magnetic material such as aluminum or titanium to save weight. Or the valve may be non-metallic. Use of an electro-magnetic or electrostatic detector is not practical in such cases.
In certain types of air guns, when the valve member is triggered, it necessarily rapidly accelerates to a relatively high velocity in order to provide the required abrupt air release. At the end of its stroke, the valve must be decelerated, usually against an air cushion that is formed in a separate chamber in the gun.
In another type of time-break detector, a pressure detector is exposed to the interior of the deceleration chamber and detects the air pressure rise in the chamber as the valve member compresses the air cushion. The detector output is displayed as an electrical transient pulse. I have discovered that because the valve member must decelerate at a finite rate, the leading edge of the transient pulse necessarily exhibits a relatively slow rise time so that the exact onset time of the pulse will tend to be fuzzy and difficult to pinpoint. I have also found that the internal pressure detector is subjected to repeated shocks in accordance with the firing-repetition rate of the gun. The lifetime of commercial piezo-electric detectors, such as is recommended in U.S. Pat. No. 4,210,222, for example, is very short. The detectors must be frequently replaced.
U.S. Pat. Nos. 3,034,827; 4,047,591; 4,210,222 and 4,240,518 are incorporated herein by reference as showings of known time-break detectors.
U.S. Pat. Nos. 3,638,752, 4,211,300 and 4,114,723 are incorporated herein by reference, and, along with the other three patents are representative of alternate air gun designs.